Nonhuman primate (NHP) models present the opportunity to test a variety of candidate AIDS vaccines, optimizing their ability to elicit immune responses and testing their ability to prevent infection or to control virus replication after virus challenge. NHP models can be used to optimize vaccines, to evaluate vaccine combinations, to determine vaccine delivery routes that provide immune responses at mucosal portals of entry, to find ways to generate broadly cross-reactive neutralizing antibodies, and, following challenge with virus, to determine whether or not the vaccine immunizations were effective. NHP models can also be used to explore a vaccines impact on the risk of acquisition of infection. NIAID supports NHP studies through both contract (Simian Vaccine Evaluation Unit contracts) and grant assistance mechanisms.